Of the chromosomal abnormalities in man, the aneuploidies resulting from errors during meiosis are of major concern. Trisomy is a leading cause of mental retardation as well as birth defects and prenatal mortality. The objectives of the proposed research are 1) to determine the effect of oocyte overripeness and underripeness, at ovulation, upon the incidence of chromosomal aneuploidy, particularly trisomy, and 2) to investigate whether advanced maternal age exerts a synergistic effect on incidence of aneuploidy after such abnormal preovulatory maturation or after chemical treatment during meiosis. We will utilize procedures which we have been developing for use in the superovulating prepubertal mouse to achieve either delayed ovulation (by 48 hours) or premature ovulation (by use of a shortened interval between follicle priming and ovulatory hormone). To determine whether maternal age influences the susceptibility of the oocyte to induced aneuploidy, adult female mice of senile or young (control) ages will be treated by methods previously found to cause aneuploidy. The number of chromosomes will be scored among early embryos. It is hoped that the information obtained in these investigations will aid in the eventual development of preventive approaches to the control of aneuploid-related birth defects.